Isaac Newton 1642 1727

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Isaac Newton 1642 - 1727
Laws of Motion Laws of Gravitation Optics
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Newtons Laws

• An object will continue at rest or at constant
  velocity as long as no resultant force acts on it

The rate of change of momentum of a body is
directly proportional to the resultant force
acting on it and in the same direction
If body A exerts a force on body B, then body B
will exert an equal and opposite force on body A
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Fundamental Forces of Nature

• Gravity- Attraction between any two massive bodies

• Electromagnetic- force between two charged
  particles

• Weak nuclear- force responsible for radioactive
  decay

• Strong nuclear- force that holds quarks together
  (constituents of protons and neutrons)

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W mg

• Weight mass ? acceleration due to gravity.

• This follows directly from F ma

• Near the surface of the Earth, g 9.81 m/s2.

• Weight is the force of gravity on a body.

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Swimming
Due to the 3rd Law, when you swim you push the
water (blue), and it pushes you back just as hard
(red) in the forward direction. The water around
your body also produces a drag force (green) on
you, pushing you in the backward direction. If
the green and red cancel out, you dont
accelerate (1st Law) and maintain a constant
velocity.
Note The blue vector is a force on the water,
not the swimmer! The green and red vectors act
on the swimmer.
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Lost in Space
Suppose an International Space Station astronaut
is on a spacewalk when his tether snaps.
Drifting away from the safety of the station,
what might he do to make it back?
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The falling apple
How could the forces on the apple be the same as
on the Earth?
apple
0.40 kg
3.92 N
A 0.40 kg apple weighs 3.92 N (W mg). The
apples weight is Earths force on it. The apple
pulls back just as hard. So, the same force acts
on both bodies. Since their mass are different,
so are their accelerations (2nd Law). The
Earths mass is so big, its acceleration is
negligible.
3.92 N
Earth
5.98 x 1024 kg
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Elephant Feather
In a vacuum, all bodies fall at the same rate.
If an elephant and a feather were dropped from a
helicopter in a vacuum (assuming the helicopter
could fly without air), theyd land at the same
time.
When theres no air resistance, size and shape
matter not!
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True or False?

• If an object is moving, there must be some force
  making it move.

False. It could be moving without accelerating.

• If v 0, then acceleration and force must be
  zero.

False. Think of a ball thrown straight up at the
top of its flight.

• An object must move in the direction of the
  resultant force.

False. It must accelerate, not move, that way.
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True or False 2

• Heavy objects must fall faster than light ones.

False. The rate is the same in a vacuum.

• When a big object collides with a little one, the
  big one hits the little one harder than the
  little one hits the big one.

False. The force on each is the same but in the
opposite direction